The invention relates to a colour dispay tube comprising an envelope having an electron gun system to generate a number of electron beams and a substantially rectangular display window comprising an upright edge and a substantially rectangular shadow mask sheet provided with a pattern of apertures and connected in the corners of the upright edge of the display window.
Such a colour display tube is disclosed in U.S. Pat. No. 3,548,235. In this patent the shadow mask does not comprise the usual rigid carrier frame. The shadow mask is formed by a substantially rectangular mask sheet manufactured from thin metal plate and having a mask ring of substantially the same thickness as the mask sheet connected to the edge. The free end of the mask ring is bent over and gives the shadow mask a certain rigidity at the sides of the shadow mask. The shadow mask is connected to the four corners of the upright edge of the display window. However, the shape of the mask ring causes moments to be exerted on the mask sheet in the case of vibrations and shocks of the tube. These moments produce deformation of the shadaw mask which causes fading of the displayed picture. Moreover, welding of the mask ring to the mask sheet is expensive and welding sputters may land on the shadow mask sheet.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a colour display tube in which the shadow mask is of a simple construction, is manufactured from the smallest possible number of components, has a large resistance against shocks and vibrations and does not substantially deform as a result of thermal effects.
It is another object of the invention to provide a screening cap which is adapted to the new shadow mask construction and which is used to screen the electron beams from the earth's magnetic field.
For that purpose, a colour display tube of a kind mentioned in the opening paragraph is characterized in that the each side of the shadow mask sheet comprises a bulged portion thereof extending toward the display window and including a collar extending away from the display window. The centre of mass of the bulged portion at any cross-section taken perpendicularly to the longitudinal direction of the portion is situated substantially in the plane of the pattern of apertures.
In a display tube in accordance with the invention the shadow mask is manufactured from one sheet. The shadow mask is rigid in its own plane. The bulged portions with collars give the sides of the shadow mask, and hence the entire mask a certain rigidity perpendicular to the plane of the shadow mask. Since the centre of mass of a cross-section of the bulged portion in a direction perpendicular to the longitudinal direction of the portion is situated substantially in the plane of the mask sheet, no moments are exerted on the mask sheet in the case of the occurrence of shocks and vibrations, so that the mask sheet remains undeformed. As a matter of fact the occurring forces lie in the plane of the mask sheet which is rigid in its own plane and are transmitted to the bulged portions where the forces act in the centres of mass of the portions.
It is to be noted that a shadow mask having a peripheral embossment extending toward the display window is known per se from U.S. Pat. No. 3,005,921. In this case, however, it concerns a post-acceleration tube having a circular shadow mask in which the peripheral embossment serves to counteract interferences of the post-accelerating field at the edge of the shadow mask.
An embodiment of a display tube in accordance with the invention is characterized in that the collar makes such an angle with the longitudinal axis of the display tube that electrons reflected by the collar fall on the mask sheet outside the pattern of apertures. As a result of this there is no need for a diaphragm, which in conventional shadow mask display tubes is connected to the upright edge of the mask sheet to prevent electrons reflected at the upright edge from landing on the mask within the pattern of apertures. Moreover, the omission of the diaphragm has the advantage that electrons land on the whole mask so that the mask is more uniformly warmed-up.
Another embodiment of the display tube is characterized in that the collar makes such an angle with the longitudinal axis of the display tube that electrons reflected by the collar are reflected in a direction which is at least perpendicular to the longitudinal axis of the tube, so that the electrons reflected at the collar do not land on the mask sheet. In this embodiment also no diaphragm is necessary since the electrons reflected by the collar do not land on the mask sheet.
Another embodiment of a display tube is characterized in that the tube comprises an internal conical magnetic screening cap which on its side facing the display window overlaps the collar of the shadow mask and extends substantially parallel to the collar. The screening cap serves to screen the electron beams from the earth's magnetic field. As a result of the overlap of the screenig cap and the collar of the shadow mask the screening cap and the shadow mask are short-circuited magnetically without the two components necessarily making mechanical contact with each other.
Another embodiment is characterized in that the overlap of the screening cap and the collar of the shadown mask is at least ten times as large as the distance between the screening cap and the collar. It has been found experimentally that good magnetic screening is obtained in this case.
Another embodiment of a display tube in accordance with the invention in which the collar makes such an angle with the longitudinal axis of the display tube that electrons reflected by the collar fall on the mask sheet outside the pattern of apertures is characterized in that the portion of the screening cap near the collar of the shadow mask comprises a shoulder which covers the aperture between the screening cap and the shadow mask. The shoulder prevents electrons between the screening cap and the collar of the shadow mask from landing on the display window via reflections.
Another embodiment of a display tube in accordance with the invention in which the collar makes such an angle with the longitudinal axis of the display tube that electrons reflected by the collar fall on the mask sheet outside the pattern of apertures is characterized in that the magnetic screening cap, in a cross-section along a plane through the longitudinal axis of the display tube, has the form of a part of an ellipse of which one focus lies in the mask sheet just outside the pattern of apertures and the other focus lies in the deflection point of the electron beam which in the direction of the edge of the pattern of apertures makes the largest angle with the longitudinal axis of the tube. The magnetic screening cap should have a shape which ensures that electrons reflected by the screening cap land on the shadow mask outside the pattern of apertures. This situation can be most effectively accomplished if the screening cap has the form of an ellipse which is rotated about the focus lying just outside the pattern of apertures in a direction towards the longitudinal axis of the display tube or if the screening cap has the form of an ellipse which is moved in a direction away from the longitudinal axis of the display tube. The impinging electrons having the largest angle of incidence will then land on the shadow mask at a larger distance outside the pattern of apertures.